I have a 1993 Volvo 850 and have issues with the ECC. The problem started with blinking REC & A/C lights upon start up. Then something blew and the smell of burnt electrics entered the cabin. I replaced the MOSFET within the power stage/ blower resistor but no change. The fan works on full speed when the headlights are on regardless of the position of the fan slider. The damper motors seem to work if I adjust them with the headlights off, and then turn the fan on by turning on the head lights.

I pulled out the ECC head unit and while I couldn't find any fried components, it did smell like the white smoke that came into the cabin which coincided with loss of control of the fan and other ECC controls.

Having read plenty of threads on similar issues, I am reasonably confident that my issue is at least 1 of the following three components - the ECC control unit, the blower resistor or the blower motor. My guess is that it is the ECC control unit based on the smell. So, the questions:

1) Of these 3 components which must be specific to right hand drive (as I live in Australia)? For example, I note that the connections on the blower resistor are on the left for left hand drive cars and on the right of the resistor on my right hand drive vehicle.
2) I understand that a bad resistor could damage the blower fan. Could a bad resistor/fan damage the ECC control unit or vice versa?

AS per the symptom and most commonly known issue on these vehicles, I would zero down on bad resistor. The smell from burnt resistor also comes from the vent becuase the resistor being mounted right in the air duct and I am picturing the possibility of smoke from vents briefly.

Thanks for the advice. It was recently serviced by a non-dealer Volvo specialist. Apparently the only error code that could not be cleared was 4-1-9 which relates to the power stage/blower resistor. So I pulled it out and replaced the MOFSET transistor as per the procedure followed by the Sydney chap on Brickboard. When testing, the symptoms did not change at all. So I decided that I should check the ECC head unit by pulling it out and taking it apart. The burning electrical smell was present on the head unit and not on the blower resistor, hence I don't think there was anything wrong with the blower resistor.

I re-checked the error codes myself, but the only code I could read was 1-2-1. Having read that the self-diagnostic system is not 100% reliable I am keen to investigate the ECC head unit further (even though my mechanic and I did not pull the error 4-2-0).

I will read your recent posts 850tony and see if there is anything further I can do to DTC test the ECC.

Any advice on the questions I asked about part compatibility between LHD and RHD?

Thanks for the assistance. It turns out I was not following the correct procedure for checking error codes. I followed Tony850's instructions more carefull and found about a dozen error codes. Code 4-2-0 was not one of them, but I am still inclined to agree with Tech that the ECC unit fried. I will post the codes later tonight to see if anyone has any further insights.

So assuming I decide to try a replacement ECC unit, do I need to get one that is designed for a right hand drive, or is the US spare part market an option for me? I'd like an answer to same question in respect of the blower resistor/power stage also as I note that the electrical connections are on the reverse side...

Oh one other question, if the blower resistor and/or blower motor are damaged, is it possible that they could cause damage to a new ECC unit? I guess I want to be sure that I don't damage a new ECC by plugging it in without replacing the blower and resistor at the same time.

I've tracked down an ECC control unit from a 1994 850. As it is a bit newer, are there any compatibility issues? My 850 is a 1993 model. The units have identical controls, but the graphics are slightly different. I think I read a comment by Tech that suggested some re-wiring may be necessary.

Also, to remove the ECC control unit from the 1994 850, is the mechanism the same as the 1993? My 1993 control unit has a spring loaded button on the underside that releases the ECC control unit from the dash when pressed. Are there any other release clips/mechanisms I should be aware of to ensure I can retrieve the ECC from the 1994 wreck?

Some 94's have the button others do not.
You might have one problem with the module in the 93. Easiest way to find out if it works or not is to put the controls on AUT if it works you are GOLDEN if it does not then you would have to run the wire. But the fan and everything will work on manual mode. All will work fine but nothing will happen on AUT.

The replacement ECC unit was really stubborn to remove, but it came eventually and without breaking the dash. Just as described by Tech.

The newer ECC unit worked exactly as described too - everything other than the AUT settings. Interestingly, the A/C and REC lights did not flash either. I thought I'd have to clear the error codes logged with the fried ECC unit first, but apparently not.

So now my diagnosis is confirmed and a replacement ECC unit is required. The one from the 1994 850 is on loan. If I can re-wire it to enable the AUT mode to work I could probably arrange to buy it. If re-wire is too difficult, I think I will try and track down the 1993 version.

Tech (or anyone else), how hard is it to re-wire the 1994 ECC unit to enable the AUT setting for use in my 1993 ECC? I tended to use AUT all the time, so I am keen to regain that function.